Ultrasonic transducer assembly for measuring liquid level

Abstract: A transducer assembly (10) for an ultrasonic liquid level measuring device utilizing a piezoelectric crystal (81) for generating ultrasonic pulses to be transmitted through a volume of liquid. The assembly (10) includes an enclosure for the piezoelectric crystal assembly (80), that defines an interior space and a sealed opening for a cable (30). The enclosure (20) is provided with a metallic layer that shields the entire interior space from electromagnetic interference. The assembly also includes a crystal housing assembly (60) and a terminal block (40) all located within the sealed enclosure (20).

Description: The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. F33657-91-C-0006 awarded by Air Force Systems Command. The invention relates to a transducer assembly for an ultrasonic liquid level measuring device, the transducer assembly including means for generating ultrasonic pulses to be transmitted through a volume of liquid, a power cable for supplying current to the means, a housing for the means, a terminal block secured to the means housing and having terminals thereon for connection to the power cable, the terminals being electrically connected to the means, and an enclosure. A known transducer assembly of this type is disclosed in GB-A-2265219. This assembly is attached to and directs ultrasonic pulses along a stillwell or tube as is well known in the art. One problem with this type of known assembly is that of eliminating the effects of electromagnetic interference (EMI) that could adversely effect the measurements obtained by the assembly. Still another problem is that of securing the enclosure that contains the ultrasonic pulse generating means to the body of the stillwell at the desired location with the ultrasonic pulse generating means positioned correctly at the end of the tube so as to direct the ultrasonic pulse along the axis of the stillwell. In accordance with the invention therefore, the transducer assembly first set forth above is characterised in that the means is a piezoelectric crystal, in that the enclosure has an opening through which the power cable extends and has interior walls with a metallic layer formed thereon to block electromagnetic interference, and in that the enclosure includes a main housing member and a cover adapted to be tightly secured to the main housing member after the crystal housing and the terminal block have been positioned in the main housing member. The transducer enclosure of the present invention reduces the problems described above and affords other features and advantages heretofore not obtainable. Fig. 1 is an elevational view showing a transducer assembly embodying the invention mounted in association with a stillwell for measuring the fuel level in an aircraft fuel tank and with parts broken away and shown in section; Fig. 2 is a side elevation taken from the line 2-2 of Fig. 1 with parts broken away and shown in section for the purpose of illustration; Fig. 3 is an end elevational view from below taken on the line 3-3 of Fig. 1; Fig. 4 is an exploded perspective view showing the various components of the transducer assembly of the invention; and Fig. 5 is a perspective view showing the crystal subassembly of the transducer assembly of the invention. Referring more particularly to the drawings, there is shown a transducer assembly 10 mounted at the bottom of a tubular probe or a stillwell 11 that is typically used in an aircraft fuel tank. The stillwell is inserted approximately vertically in the tank. The purpose of the stillwell is to provide a liquid column that represents the level in the entire fuel tank but which is not subject to turbulence and other influences that could adversely effect the measurement. An ultrasonic transducer located in the transducer assembly 10, transmits a sinusoidal acoustical pulse through the stillwell 11 to the surface of the liquid. An echo returns from the surface of the liquid to the transducer and provides a sinusoidal echo signal. A receiver responds to the echo signal so that the total elapsed time from transmission to reception may be determined. Since the elapsed time is a function of the height of the column of liquid in the stillwell, the level of fluid in the tank may be readily determined using techniques well known to those skilled in the art. The calculated data may then be displayed. The transducer assembly 10 includes as its principal components an enclosure 20, a power cable 30, a terminal block assembly 40 and a crystal housing assembly 60. The enclosure 20 includes a main housing 21 and a cover 22 both formed of a suitable plastic material. The housing and cover, when assembled, define an interior space in which the various components of the transducer assembly are mounted. Because the assembly 10 is emersed in a fuel tank, it is necessary that the enclosure be tightly sealed to prevent intrusion of the fuel into the interior. Also, the cover 22 is removably secured to the main housing 21 to permit convenient removal if access to the interior components is required. The main housing 21, is provided with a pair of laterally extending arms 23 and 24 that have a curved configuration, the curve being defined by an arc centered at a longitudinal axis between the two arms. The arms 23 and 24 are used to secure the transducer assembly 10 to the stillwell 11 in the manner shown in Fig. 1. The arms are flexible and resilient so that the enclosure 20 need be merely pressed against the wall of the stillwell 11 to flex the arms outwardly sufficiently to permit the enclosure to be moved into position. Then the arms flex inwardly to grip the stillwell and provide a secure connection. It will be noted that this arrangement permits the transducer assembly to be quickly secured and removed from the stillwell as necessary. The main housing 21 also has a laterally extending portion 25 in which the crystal housing assembly 60 is mounted and which is adapted to be placed at the bottom open end of the stillwell 11 in a position so that an ultrasonic pulse generated by the transducer assembly 60 may be directed upwardly along the axis of the stillwell to the surface of the liquid. In accordance with the invention, the interior surfaces of the enclosure 20 are coated or plated with metallic material to shield the interior components from electromagnetic interference which is often present in an aircraft fuel tank environment. The conductive shielding for the enclosure 20 is accomplished by coating the parts with electroless copper followed by electroless nickel for corrosion resistance. The main housing 21 and cover 22 are preferably coated on both sides although conductive coating on one side only would produce sufficient shielding. Coating the entire inside and outside surfaces, however, is simpler and less costly. To provide shield continuity between the main housing member and the cover, a raised contact strip 26 is provided on the engaging surfaces of the main housing. It will be seen from Fig. 4, that when the main housing 21 and cover 22 are assembled, they define an opening 27 for the power cable 30. The opening is formed by means of a semi-circular cut-out 28 formed in the main housing 21 and a slot 29 formed in the cover 22. When the cover and main housing are assembled, the inner end of the slot 29 and the cut-out 28 define the circular opening 27. The cable 30 extends through a ferrule 31 to provide strain relief and a tight seal. The ferrule 31 fits tightly in the opening 27 and is in fact electrically bonded to the metallic plating around its entire circumference. Also, the ferrule 31 is preferably formed of an electrically conductive material so as to extend the electromagnetic interference shield through the space it fills in the opening 27. The cable has a pair of conductors 33 and 34 to provide current to the crystal as will to be described below. The terminal block assembly 40 includes a molded member 41 formed of a suitable plastic material and having three parallel walls 42, 43 and 44 that serve to define a pair of channels 45 and 46. The channels have openings formed in the floor thereof that extend through the block. A pair of terminal elements 47 and 48 are mounted in the openings and are adapted to receive the conductors 33 and 34 from the cable 30. The crystal housing assembly 60 is received, for the most part, in the extending portion 25 of the main housing 21 and is intended to provide a secure mount for the crystal assembly 80. The housing assembly includes a housing block 61 preferably formed of molded plastic material. The block has a relatively flat shape with a parametric sidewall that defines a recess 62. The recess 62 has a floor 63 with a pair of openings 65 and 66 formed therein. The openings 65 and 66 are adapted to receive a pair of electromagnetic interference filters 67 and 68 that extend axially therethrough. The floor 63 also has a circular well 70 formed therein adapted to receive and support the crystal assembly 80. Also a pair of resistors 71 and 72 are mounted in the recess 62. The resistors are connected in parallel with the electrodes for the crystal assembly 80 to provide a redundant charge drain path. The crystal assembly 80 includes a piezoelectric crystal 81 which may be, for example, a lead-zirconate-titanate piezoelectric ceramic in the form of a disc with electrodes on each face to produce a roughly planar compressive wave. The crystal 81 is acoustically matched to the fuel by means of a quarter-wave-thickness face plate or window 82 with acoustic impedance chosen for optimum energy transfer. The quarter-wave construction also minimizes internal vibrations to provide maximum resolution and accuracy. The rearward side of the crystal is acoustically isolated for maximum sensitivity and to prevent emission of sound from the bottom of the transducer and subsequent false echoes from the tank bottom. The isolation is provided by means of a back piece 83. The crystal is adhesively bonded to the face plate 82. Acoustic transparency of the bond is attained by suitable flatness of the surfaces, extensive cleaning, application of primer to the electrode surface, degassing of the adhesive and curing under uniform pressure. The piezoelectric crystal 81 is inherently weak mechanically in tension but strong in compression. To make optimum use of this property, the transducer is designed to surround the crystal with window material symmetrically so as to minimize stresses in the crystal due to differential thermal expansion of the crystal and window material. This minimisation of stress is achieved by the crystal assembly 80 described above consisting of the window 82 and backing plate 83. These are adhesively bonded around the crystal 81 with an angled joint (Fig. 1) designed to minimize adhesive stresses. A pair of electrodes 85 and 86 from the bonded face are brought to the opposite face to facilitate electrical connection. A tab on the bonded face is flattened to match the electrode surface. Also, the electrical connections to the crystal may be made with conductive adhesive to protect the crystal from damage from soldering heat. The backing plate 83 has a relief for acoustical isolation and has a passage for the electrical leads. Accordingly, the backing plate 83 is not dimensionally symmetrical to the window 82 but is sized to be mechanically symmetrical in its stiffness and thermal expansion characteristics. The crystal assembly 80 is inserted into the crystal housing assembly 60 and secured in place with conductive adhesive to ensure shield continuity. The terminal block assembly 40 and the crystal housing assembly 60 are bonded together before assembly into the main housing member 21. It will be seen with reference Fig. 4 that the assembly of the two components 40 and 60 can easily be accomplished to form an integral unit that may then be inserted in the main housing 21 and bonded in position. Then the leads 33 and 34 from the power cable may be connected to the terminals 49 and 50. After this, the cover 22 may be secured to the main housing 21 to complete the assembly. This arrangement provides a transducer assembly with substantial shielding against electromagnetic interference to improve the accuracy of the ultrasonic liquid level measuring system. While the invention has been shown and described with respect to a specific embodiment thereof, this is for the purpose of illustration rather than limitation and other variations and modifications of the specific device herein shown and described will be apparent to those skilled in the art all within the scope of the invention as defined in the appended claims.

Claims: A transducer assembly (10) for an ultrasonic liquid level measuring device, the transducer assembly including means (81) for generating ultrasonic pulses to be transmitted through a volume of liquid, a power cable (30) for supplying current to the means (81), a housing (60) for the means (81), a terminal block (40) secured to the means housing (60) and having terminals (47,48) thereon for connection to the power cable (30), the terminals being electrically connected to the means (81), and an enclosure (20), characterised in that the means is a piezoelectric crystal (81), in that the enclosure (20) has an opening (28,29) through which the power cable (30) extends and has interior walls with a metallic layer formed thereon to block electromagnetic interference, and in that the enclosure (20) includes a main housing member (21) and a cover (22) adapted to be tightly secured to the main housing member (21) after the crystal housing (60) and the terminal block (40) have been positioned in the main housing member. A transducer assembly as defined in claim 1, characterised by means (31) providing a seal between the power cable (30) and the edges of the opening (28,29). A transducer assembly as defined in claim 2, characterised in that the sealing means comprises a tubular ferrule (31) formed of electrically conductive material and having a portion thereof bonded to the metallic layer. A transducer assembly as defined in claim 1, characterised in that the enclosure (20) includes means (23,24) for attaching the assembly (10) to a stillwell (11). A transducer assembly as defined in claim 4, characterised in that the attaching means comprises a pair of flexible resilient arms (23,24) extending outwardly from the enclosure (20) and adapted to flex outwardly during attachment and to move into a holding position to surround and grip the stillwell (11) therebetween. A transducer assembly as defined in claim 1, characterised in that the main housing member (21) has an outwardly extending portion (26) at one end that is adapted to contain the crystal housing (60), the extending portion being adapted to be positioned at the bottom end of a stillwell (11) whereby the crystal (81) is oriented to direct ultrasonic pulses along the stillwell axis. A transducer assembly as defined in claim 1, characterised in that the piezoelectric crystal (81) is assembled in combination with a window element (82) and a back plate (83), the window element having an acoustic impedance with efficient energy transfer characteristics and the back plate being adapted to isolate the crystal from undesired acoustic echoes. A transducer assembly as defined in claim 7, characterised in that the crystal (81) is secured between the window element (82) and the back plate (83) by adhesive bonding, the circumferential portions of the window element (82) and the back plate (83) extending radially outward beyond the circumference of the crystal (81) and being in engagement with one another at an angled joint to minimize stresses in the crystal due to differential thermal expansion.

Inventors: BIRKETT ROBERT E, BIRKETT, ROBERT E.
Applicants: SIMMONDS PRECISION PRODUCTS, SIMMONDS PRECISION PRODUCTS INC.
Application year: 1994
Main classification: G01F23
IPC codes: G01F23, G01F15, H05K9